A reflective display device can carry out a display by reflecting externally entered light. This allows an improvement in low power consumption, a reduction in thickness, and a reduction in weight. On the other hand, a transparent display device can be used in an environment of weak surrounding light.
Given this factor, as a display device having both advantages of a reflective display device and a transparent display device, a semi-transparent display device, in which both a transparent display unit and a reflective display unit are provided in a single pixel, has been developed recently. Such a semi-transparent display device realizes a fine display with low power consumption by carrying out a reflection display with the use of externally entered light in a bright surrounding environment, whereas realizes a fine display by carrying out a transmission display with the use a backlight in a dark surrounding environment.
In addition, there has been a transparent display device adopting an IPS (In Plane Switching) system as a technique for realizing a wide viewing angle. A transparent display device adopting a vertical electric field driving method is inferior in viewing angle property to one adopting a horizontal electric field driving method.
In view of the circumstances, it has been suggested to utilize a semi-transparent display device adopting the IPS method in order for the semi-transparent display device to further improve the viewing angle property.
For example, (1) a semi-transparent liquid crystal display device using a comb-teeth electrode as a reflective electrode (see Patent Document 1, for example), and (2) a semi-transparent liquid crystal display device in which a comb-teeth electrode and a reflective member are separately provided (see Patent Document 2, for example) are known as the semi-transparent display device adopting the IPS method as the horizontal electric field driving method.
As an example of the semi-transparent display device, FIG. 6 and FIG. 7 are cross-sectional views showing schematic structures of main parts of semi-transparent liquid crystal display devices, adopting the IPS method, disclosed in Patent Documents 1 and 2, respectively.
As shown in FIG. 6, the semitransparent liquid crystal display device, adopting the IPS method, disclosed in Patent Document 1 utilizes, as a reflective electrode in a reflective display unit 55 (a reflective region), a comb-teeth electrode 54 provided on a backside substrate (hereinafter referred to as “lower substrate”) 53 that is provided on the opposite side of a display-side substrate (hereinafter referred to as “upper substrate”) 52, a liquid crystal layer 51 being sandwiched by the upper and lower substrates 52 and 53. A Gap section between neighboring comb-teeth electrodes 54 is used as a transparent display unit 56 (a transparent region). The comb-teeth electrode 54 is a reflective pixel electrode via which the liquid crystal layer 51 sandwiched by the lower substrate 53 and the upper substrate 52 is driven. In the transparent display unit 56, liquid crystal molecules 57 change their alignment state in accordance with an electric field caused by the comb-teeth electrode 54.
As shown in FIG. 7, the semitransparent liquid crystal display device, adopting the IPS system, disclosed in Patent Citation 2 includes a pixel electrode 69 and a common electrode 70, serving as a comb-teeth electrode, via which a horizontal electric field driving is carried out, in a reflective display unit 71 (a reflective region) and in a transparent display unit 72 (a transparent region), respectively. An electrical field generated between the pixel electrode 69 and the common electrode 70 drives a liquid crystal layer 63 sandwiched by a lower substrate 61 (a backside substrate) and an upper substrate 62 (a display-side substrate).
With the semi-transparent liquid crystal display device, in the lower substrate 61 of the transparent display unit 72, the pixel electrode 69 and the common electrode 70 are arranged so as to be parallel to each other on a transparent insulating substrate 64, via a first insulating film 65. Meanwhile, in the reflective display unit 71, a second insulating film 66, a reflective plate 67 (a reflective member), and a third insulating film 68 are stacked on the first insulating film 65 in this order. The pixel electrode 69 and the common electrode 70 are provided on so as to be parallel to each other. The second insulating film 66 and the third insulating film 68, by which the reflective plate 67 is sandwiched, are provided for adjusting thicknesses of the liquid crystal layer 63 in the reflective display unit 71 and in the transparent display unit 72, based on a difference in optical path lengths between the display units 71 and 72.    [Patent Document 1]Japanese Unexamined Patent Publication No. 11-242226 (publication date: Sep. 7, 1999) (U.S. Pat. No. 6,281,952 (issue date: Aug. 28, 2001))    [Patent Document 2]Japanese Unexamined Publication No. 2003-344837 (publication date: Dec. 3, 2003) (U.S. Pat. No. 6,914,656 (issue date: Jul. 5, 2005))